Global positioning systems (GPS) provide satellite based location services in devices such as smartphones for applications such as vehicle navigation. However, GPS has several limitations which prevent practical application in other areas. For example, GPS satellite services work less well indoors due to the attenuation of the GPS radio signals. Furthermore, the accuracy provided by GPS is on the scale of approximately ten meters. While sufficient in applications such as vehicle navigation, this accuracy is insufficient in other applications. For example, in many indoor use applications, accuracy on the scale of approximately one meter is desired.
Received signal strength indication (RSSI) based location techniques have been used. As the radio signals propagate, their power strength decreases as the distance increases. When the transmit power is known, the received power gives an indication about the distance travelled. However, RSSI provides only a distance. In order to obtain a position, multiple transmitters with known locations are needed to perform triangulation or trilateration. Furthermore, the RSSI measurements are heavily disturbed by environmental effects such as multipath fading and shadow fading. In particular, shadow fading, which is caused by additional attenuation in the signal path (e.g., a human body), makes the RSSI technique quite inaccurate for positioning applications.
As a result, improved methods and apparatuses for device location are needed.